How to Mix Face Textures Across UV Edges
Here is an example to illustrate how you can use Empties to control some aspect of your material - in your case textures. This is how it is done in Cycles, so hopefully you are able to carry this concept over into your game code.

Just a basic black and white spherical gradient appearing on a simple landscape object. The placement and size of the gradient are determined by the Empty (it's located where the 3D Cursor is in the screen shot). The Color Ramp is for controlling the falloff.



And here is an example of what this technique can produce when you have multiple Empties and they are actually controlling textures instead of just a color result. In the screen shot below two Empties are used against a base grass texture.